Abstract
The virtually untapped Floridan aquifer system is considered to be a
supplemental source of water for public use in the highly populated
coastal area of Palm Beach County. A recent study was conducted to
delineate the distribution of salinity in relation to the local
hydrogeology and assess the potential processes that might control
(or have affected) the distribution of salinity in the Floridan aquifer
system.

The Floridan aquifer system in the study area consists of the Upper
Floridan aquifer, middle confining unit, and Lower Floridan aquifer
and ranges in age from Paleocene to Oligocene. Included at its top is
part of a lowermost Hawthorn Group unit referred to as the basal
Hawthorn unit. The thickness of this basal unit is variable, ranging
from about 30 to 355 feet; areas where this unit is thick were
paleotopographic lows during deposition of the unit. The uppermost
permeable zones in the Upper Floridan aquifer occur in close association
with an unconformity at the base of the Hawthorn Group; however, the
highest of these zones can be up in the basal unit. A dolomite unit of
Eocene age generally marks the top of the Lower Floridan aquifer, but
the top of this dolomite unit has a considerable altitude range: from
about 1,200 to 2,300 feet below sea level. Additionally, where the
dolomite unit is thick, its top is high and the middle confining unit
of the Floridan aquifer system, as normally defined, probably is not
present.

An upper zone of brackish water and a lower zone of water with salinity
similar to that of seawater (saline-water zone) are present in the
Floridan aquifer system. The brackish-water and saline-water zones
are separated by a transition zone (typically 100 to 200 feet thick)
in which salinity rapidly increases with depth. The transition zone
was defined by using a salinity of 10,000 mg/L (milligrams per liter)
of dissolved-solids concentration (about 5,240 mg/L of chloride
concentration) at its top and 35,000 mg/L of dissolved-solids
concentration (about 18,900 mg/L of chloride concentration) at its
base. The base of the brackish-water zone and the top of the
saline-water zone were approximately determined mostly by means
of resistivity geophysical logs. The base of the brackish-water
zone in the study area ranges from about 1,600 feet below sea level
near the coast to almost 2,200 feet below sea level in extreme
southwestern Palm Beach County. In an area that is peripheral to
Lake Okeechobee, the boundary unexpectedly rises to perhaps as
shallow as 1,800 feet below sea level.

In an upper interval of the brackish-water zone within the Upper
Floridan aquifer, chloride concentration of water ranges from 490
to 8,000 mg/L. Chloride concentration correlates with the altitude
of the basal contact of the Hawthorn Group, with concentration
increasing as the altitude of this contact decreases. Several
areas of anomalous salinity where chloride concentration in this
upper interval is greater than 3,000 mg/L occur near the coast.
In most of these areas, salinity was found to decrease with depth
from the upper interval to a lower interval within the brackish-water
zone: a reversal of the normal salinity trend within the zone. These
areas are also characterized by an anomalously low altitude of the
base of the brackish-water zone, and a much greater thickness of the
transition zone than normal. These anomalies could be the result of
seawater preferentially invading zones of higher permeability in the
Upper Floridan aquifer during Pleistocene high stands of sea level
and incomplete flushing of this high salinity water by the
present-day flow system.